Several chemical analysis are done by automatic analyzing equipment, in hospitals and laboratories. For the analysis amounts of liquid sample medium and reagents have to be automatically measured into analyzing vessels. The amounts to be measured are typically very small, in the range of 1-250 μl and the amount to be displaced and transferred must be very accurately controlled. One typical way to meter and transfer liquids is to use a suction tube or needle wherein the amount of sample or reagent is sucked and wherefrom it is ejected to a desired vessel. The pressure differences required for sucking and ejecting the liquid are effected by an accurate metering syringe. The proportioning and transfer of liquids is one of the most vulnerable functions of automatic analyzers and very dependent on the properties of the liquid to be handled. Possible disturbances causing errors in analyzes are deviances in the properties of the sample or reagent like clogging or abnormally high viscosities, failed sucking of the liquid leading to an insufficient or missing metering. Missing suction can be caused by foaming of the sample or by other failures in detection of the surface of the sample.
Detecting the failures in metering is essential to reliable functioning of the apparatus. Since this type of analyzers are often used in medical analysis for diagnosing diseases, no faulty analyzing results can be allowed in any of the samples. Rejection of a correct metering is very undesirable for example since the volumes of the samples are small and not easily replaceable. When the analyzes are used for diagnosing diseases, interpreting a failed metering as a successful one can be even catastrophic.
Presently the metering is controlled by measuring continuously the pressure in the suction line of the metering apparatus. Measurement is done by an accurate and therefore expensive pressure gauge that is connected to the suction line. The gauge can be flow-through type or connected to a branch from the suction line. The control of a successful metering is based on surveillance of the pressure during the metering. The peak values of the pressures during suction and ejection as well as recovery times after process steps are controlled and different algorithms and threshold values are used for determining whether the metering has been successful or not. In some methods surface integration of the pressure functions are used for evaluation by comparing them to calibration values.
Above-mentioned methods are described in patents EP 0981048, U.S. Pat. No. 6,370,942, WO 9208545, EP 0169071, EP 0571100, U.S. Pat. No. 4,780,833 and EP 0289946, for example. A common feature of these apparatuses is the surveillance of the suction or ejection of the liquid and pressure changes related thereto.
The above-mentioned methods are most suitable for detecting clogging of the metering line. These types of methods have anyway several drawbacks. The methods require quite tedious calibration and complex algorithms determining the success of the metering act. This rises the probability of faulty interpretations and maintenance of the system becomes difficult and time-consuming. Since one multifunctional automatic analyzer utilizes normally several metering volumes and different metering cycles and every combination of these has a characteristic pressure curve, it is very tedious to accomplish a valid general calibration and threshold values. Varying viscosities of the liquids complicate the matter even further.
Even further problems are caused by pressure oscillations that is caused by rapid acceleration or deceleration of the metering needle or tube. This causes the liquid to move within the metering line, which of course causes pressure variations in the line. This oscillation effectively disturbs the pressure signal that is monitored. It would be easy to avoid this problem by lengthening the cycle times of the apparatus so that the oscillation is dampened. This is not possible since a fast operation and output is required of these apparatuses whereby it is not affordable to use unnecessary waiting times. For this reason the signal must be filtered. This requires more complicated programming and may cause loss of data and lead to faulty results. On small metering volumes like 1 or 2 μl the changes in the measured pressure signal are weak and they tend to be covered by disturbances in the measured signal. Most apparatuses cannot control the metering of such a small volumes.
The mechanical condition of the metering line cannot normally be detected by these methods. Therefore it is examined separately by following the washing pressures of the line in order to detect leaks and other possible mechanical faults. It must be noted that leakage of the metering line leads to faulty metering that can be left unnoticed when these methods are used.
The above described methods yield lost of data on the measurement act, but that data is in such a format that it cannot be easily and reliably used for simply detecting the success of a single metering act. The pressure information over the whole metering act is not needed. It would be desirable to have a method wherein a simple indication of a faulty metering is reliably obtained.